I know this answer comes late, but perhaps it will add additional info for folks coming here for the first time.
The weight of the glove is not related to the hand size or amount of protection that it offers. Once you know the weight you want, you'll want to try on a few different brands to see what works for your hands - I, for instance, swear by my Twins because they are a great fit for my small hands and short forearms, while others may find them too snug.
The regulations for competition vary by region so I would suggest googling for your region's boxing association website which is sure to clarify which is the right weight for competing in your area.
In Ontario the competition weight for amateurs is 10oz, and these gloves must also have a white stripe and the AIBA label of approval.
For professionals, 6, 8, and 10 oz. gloves could be used it, varies by jurisdiction so you'd really need to ask your coach or regional association which is the appropriate weight for you.
As the others have mentioned, 16 oz is generally the recommendation for sparring - though younger youth or very light-weight individuals may prefer 14oz.
When I first started out 18 years ago my coach's recommendation was to train with 16 oz gloves, however I disagree with this approach. While it can be great for building muscle endurance, the high weight over a long stretch of time can be a strain on the shoulders and cause muscle fatigue (I personally was never able to adjust to the weight and was in chronic pain until I switched to training in 12 oz gloves.) Consider alternating weights for this reason - do your speed and technical bag training in bag gloves (10 or 12 oz depending on your weight) and add in some shadow boxing with 1 or 2 lb hand weights into your routine to strengthen your shoulders.
Another reason you won't want to use your sparring gloves for bag-work - you'll wear them out a lot quicker, so even if you do choose to train at that weight, you'll still want to consider a second pair.